Electric discharge machines which utilize electrodes and electrical current for cutting calibration precision notches into hard metals are well known. The commercially available equipment, however, involves an expensively constructed non-portable apparatus which utilizes a cantilevered probe where an electrode is mounted on the end of the probe. A fixture holds a tubular pipe section. The electrode is movable by the fixture to bring the electrode into proximity with the inner wall of a pipe or other work material so that an electrical arc does a cutting operation. Thereafter, only the electrode is moved during the cutting of a notch and the pipe or work material remains stationary. The electrical current for the arc is applied via the electrode and the pipe.
This equipment and process as described above can be used to produce what is known as an API (American Petroleum Institute) notch in tubular pipe where the pipe is used in drilling and oil field use. The notch in the pipe is a standard notch with precise measurements so that the notch can be used to calibrate the measurements of an electromagnetic inspection device or a sonic inspection device. The purpose of the notch is to provide a measured or known reference loss of wall thickness value in the pipe wall for the testing equipment. Thus, when the testing equipment is utilized to inspect the wall of the pipe, the measurements of wall thickness or wall integrity obtained by the equipment can be calibrated with respect to the measured loss of wall thickness caused by the notch. For example, it is typical to cut an external notch in a pipe wall with a saw blade to a depth equal to 5% of the wall thickness of the pipe. The depth of the notch and the wall thickness of a pipe, of course, can be measured. Thereafter the measurement equipment for wall thickness can detect the notch and should give a measurement relative to the known decrease in wall thickness of 5%. This relative measurement value can be used for a sensitivity adjustment with respect to the equipment so that the subsequent measurements by the equipment are, in effect, calibrated to a known reference on the pipe itself.
While, heretofore, the API standards for calibration have indicated that there should be internal notches within the bore of the pipe as well as external notches for calibration, the method presently used to obtain an internal notch is to cut a pipe into short sections, of say, eight feet in length from a string of pipe; send the sections off to a location where an internal notch cutting apparatus is located; and then cut notches in each section. After the notches are cut in each section, the cut sections of pipe are reassembled by welding for inspection calibration purposes. Obviously, the entire process involved to form internal notches in this manner is expensive and considerable care and expertise is required for the reconstruction consistent with current inspection techniques.
There is also a problem in the fact that the type of notches required for calibration should have different orientations, i.e., be longitudinal to the length of the pipe, transverse to the length of the pipe, and at an angle or angles with respect to the longitudinal axis of the pipe. The need for transverse and angular cuts in outer pipe surfaces cannot be easily solved with saw blades which are not easily adaptable to making cuts of uniform depth in these directions and precision internal angular notches cannot be cut with saw blades by contemporary methods and apparatus.